The present invention relates to soft tissue or bone-to-bone fixation devices, and more particularly to those comprised from the biocompatible polymer polyether-ether-ketone (PEEK).
Current biocompatible polymeric screws, such as those comprised of poly-1-lactide acid (PLLA) or poly-lactide acid (PLA), lack the durability to be inserted into cancellous bone without the additional step of manually tapping the implant site intended to receive the bone screw with a surgical bone tap. Though screws created from these materials possess the desired characteristics of radiolucency and MRI compatibility, the additional procedural step of tapping is often described as difficult, and translates into increased operating time.
There are relatively few tissue fixation devices in use in the orthopedics industry which are fabricated of PEEK, and there are no implantable PEEK bone screws available on the market. Within the industry, the trend is away from polymer-based and toward composite interference screws. For example, the thinking in the industry is that PEEK interference screws would have a higher likelihood of loosening and, thus, a relatively low pull-out strength, and they are thus disfavored for this type of application.
A typical application for bone screws such as the one described herein is in the reconstruction of the anterior cruciate ligament (ACL). This procedure is performed either with a soft-tissue or a bone-patellar-tendon-bone (BPTB) graft, currently considered to be the “gold standard” of ACL reconstruction procedures by many orthopedic surgeons. It is intended that the disclosed fixation implant is packaged sterile and used in conjunction with a reusable driver.
Current non-metallic interference screws pose several challenges for the patients who have them implanted and for the surgeons who utilize them. These challenges include, for example, a lack of circulation in bone and therefore slow or no absorption of bioabsorbables; weak tips, fracturing, and thread blunting; a requirement of tapping bone prior to insertion, as discussed above; and tendency of non-metallic screws to pull out of the bone site due to applied loading during therapy or recovery.
There are currently many different interference screws marketed for soft tissue or bone-tendon-patellar-bone ACL reconstruction. A bioabsorbable screw available from ARTHREX is indicated for use in both types of reconstruction and is comprised of PLLA. The blunt threads of the ARTHREX screw facilitate its use with soft tissue grafts by avoiding the problem of tissue laceration that could occur with sharper threads. However, because of these blunt threads, the ARTHREX Biointerference and Biocomposite Interference Screw Instructions for Use (IFU) state: “Prepare screw entrance insertion point using either the tap, dilator, or notcher. (Use only a dilator or notcher for the Round Delta Bio-Interference Screw.)”.
The DePuy Mitek Biocryl screw is a composite made from the osteoconductive ceramic tricalcium phosphate (TCP) and polylactic acid (PLA). The DePuy Mitek Milagro screw is a composite made from the osteoconductive ceramic tricalcium phosphate (TCP) and poly(lactic-co-glycolic acid) (PLGA). These screws require a starter tap to “help screw engage in cortical bone.” The Mitek IFU for both screws states: “Pre-tapping is necessary when inserting the interference screw into hard cortical bone with bonetendon grafts. The starter tap should be used no more than three to four full turns. Advancing the starter tap further may jeopardize fixation integrity.”
Fixation strengths of interference screws are comparable between similarly sized screws across brands and product offerings. Many, if not all, bioabsorbable or biocomposite screws require a separate tapping procedure to prepare the implant site for screw insertion in harder bone to prevent damaging the screw. Many other screws require the use of a notcher or dilator to prepare the insertion site for the screw. Avoidance of such additional preparatory steps at the implantation site would be a significant advantage to practitioners in this medical field.